1. Field of the Invention
The present invention pertains to a method for the removal of surface coatings from various surfaces. In particular, this invention pertains to a method of using ultra-high pressure water to remove surface coatings, including paint, to expose the metal hulls of ships. A remotely controlled platform having a ferro-magnetic and motive means moves the ultra-high pressure nozzles about the surface to be treated. An alternate embodiment of the invention incorporates a recycling and waste disposal system whereby the water is recovered, the coating particulate removed and the water reused as an abrasive.
2. Description of the Prior Art
The marine environment is extremely demanding on coatings applied to hulls, or other bodies, which are submerged for long periods of time. The corrosive properties of salt water are well known, and in this hostile environment even the most durable coating must be replaced periodically.
The degree of surface roughness of submerged portions of ships has a great effect on both ship fuel efficiency and the speed which can be achieved at a given propeller revolution rate. Roughness can be caused by marine growth ("fouling"), degradation of hull coatings, and deterioration of unpainted surfaces such as propeller blades. For commercial, private or military ships, losses in ship performance can have a variety of consequences, both financial and in terms of meeting scheduled arrival dates.
Although the following examples are for a VLCC Very Large Crude Carrier; an oil tanker, with the following typical approximate specification: 272,000 tons deadweight; total engine horsepower (at 90 RPM propeller rate): 32,700 hp, examples could be given for any size or type of marine craft. A typical trip for a VLCC is from the U.S. Gulf Coast to the eastern end of the Mediterranean Sea. This round trip normally takes about 40 days. However, with an increased surface roughness causing a loss in peak speed of only 1 knot (nautical mile per hour), 21/2 days would be added to the trip.
Considering the effect of surface roughness on efficiency, for a VLCC, each increase of 1 RPM in propeller rotation rate corresponds to an increase in ship speed of about 0.15 knot. Thus, a roughness caused loss of one knot would require an increase of about 6.7 RPM to maintain the same ship speed (i.e., to overcome the increased ship resistance). This increased propeller speed requires 20 tons (metric ton) per day of extra fuel.
Marine engineers estimate that an increase in the average roughness of a ship's hull of about 30 microns (peak-to-peak, RMS roughness) can cause a drop in peak achievable speed of about one percent. A new hull can have a surface roughness of about 160 micron and a deteriorating coating can be about 280 micron. This roughness increase could cause a four percent drop, which for a typical 16 knot VLCC peak speed is a loss of about 0.64 knots.
The foregoing clearly demonstrates the economic importance of maintaining the submerged surfaces of ships in as smooth a condition as is practical. Therefore providing a means to maintain surface smoothness of ships is a practical and economical objective for ship owners.
Fouling of ship bottoms not only reduces fuel efficiency, thus increasing operating costs, but also attacks the integrity of the coating which leads to corrosion and metal fatigue. Corrosion damage to hulls can lead to costly repairs, loss of operating time and, if unchecked, to the premature scrapping of the vessel. Environmental laws hamper fouling prevention by limiting the types of paint which may be used, especially those containing organotin/tributylin and cuprous oxides which are most effective in controlling calcerous fouling. Thus, because the most effective preventive measures against fouling are unavailable it has become necessary to replace coatings more frequently. The coatings which can be applied under current laws need a superior surface finish in order to extend the life of the coating on the surface. Astute ship owners realize a superior surface finish extends the life of the coatings and reduces drydock time and expenses in the future.
There are presently three existing cleaning methods which are used for cleaning ship's hulls:
1. Chemical paint strippers are currently used to remove small patches on a ship's hull, as is required for non-destructive testing and access cuts. This method is unsuitable for cleaning the entire ship's hull and it creates large amounts of toxic waste for each area cleared; PA1 2. Abrasive grit blasting is used for cleaning the entire hull. Current hull cleaning methods using abrasive grit generate tons of abrasive that must be detoxified if used on the organic paint, and millions of gallons of water that must be treated annually. Satisfactory methods have not been developed to manage the organic waste during the application of the paint, removal of spent paints in dry-dock prior to repainting, and detoxification of the grit and other wastes generated during the present abrasive blasting method used; and PA1 3. High pressure water jet systems, which use water pressures of less than 10,000 psi are usually applied by a hand held nozzle. The prior art water nozzles lacked sufficient pressure, typically less than 10,000 PSI, to completely remove paint from the surface of a hull. Another major disadvantage of prior art rotary water jets is the slow rate at which multiple layers, or very hard coatings, can be removed.
To fully remove coatings from hulls it is necessary to construct platforms for operators from wooden bars or rods, or by suspending gondolas or cradles or the like from above, when blasting. Thus, the operators must work by hand on such platforms located at a considerable height. For this reason, the operation is dangerous and, in addition to this, the efficiency of removal is extremely low. To overcome this disadvantage, various steered vehicles have been proposed. However, such vehicles heretofore proposed have proved to be unsatisfactory, for example, in that when the vehicle is driven over a surface having a certain curvature instead of a flat plane of a structural member, it is not possible to obtain a sufficient adhesion force for supporting its weight and further that it tended to sometimes damage the operation surface due to the contact pressure exerted between the vehicle and the operation surface.
Some vehicles use permanent magnet means and/or electromagnet means mounted thereon, and are driven by magnetic belts provided on both sides of the vehicle. Changing the direction of travel of a vehicle of this type requires remarkable skill as the apparatus has the tendency of slipping at the contact area either of the right or left belts during the turn of the vehicle owing to the change in the contact pressure between the vehicle and the wall surface.
Some examples of the prior art are:
Hirosha et al U.S. Pat. No. 3,777,834 discloses a magnet vehicle supportedly adhered to and adapted to run over an inclined wall surface of a structural member made of a strong magnetic material such as iron and steel, characterized in that the vehicle can be altered of its direction of travel as desired on the surface.
Woods U.S. Pat. No. 3,922,991 discloses an apparatus for cleaning relatively large, flat, ferro-metallic surfaces of corrosion, scale, paint and undesirable marine growths, the apparatus having high pressure fluid blasting assembly, magnetic attachment device, driving motor, and a signal generating and receiving system for guiding the apparatus along the surface to be cleaned.
Cadutt U.S. Pat. No. 4,890,567 discloses a robotically operated device using an ultrasonic transducer for the cleaning of ships' hulls. The device may also be used for spraying paints or other chemicals on the sides of ships' hulls. The device includes a housing having an open face adapted to confront a ship's hull and apparatus disposed in the housing for impinging a flow of fluid through the open face onto the ship's hull. An ultrasonic transducer is disposed in the housing for impinging a flow of ultrasonic energy through the open face onto the ship's hull. Apparatus connected to the outside of the housing retains the housing on the ship's hull and moves the housing on the ship's hull. In an additional embodiment, apparatus for spraying paint or other chemicals on a ship's hull is disposed in the housing.
Andosen U.S. Pat. No. 4,926,775 discloses a cleaning apparatus for use under water, particularly for cleaning vertical surfaces which are fouled by marine organisms. One or more nozzles for spraying water at high pressure at a surface to be cleaned are arranged on a rotary disc-shaped unit where the rotation axis is intended to be generally perpendicular to the surface which is to be cleaned. The nozzles are obliquely located in a circular plane so that the rotating unit can rotate. Beyond the periphery of the rotating unit there is a casing which forms an annular chamber with an outlet which is at least partly directed away from the surface which is to be cleaned. The annular chamber is formed from two generally cylindrical or truncated conical casing units which are positioned at a radial distance from each other with their internal ends concentric with respect to the nozzle holder.
Rabuse U.S. Pat. No. 4,690,092 discloses an aquatic scrubbing device for attachment to an underwater ferro-magnetic surface incorporates a carriage, at least two independently energizable electromagnets supported by the carriage for rotation about mutually parallel axes, and at least one drive motor for rotating the electromagnets relative to the carriage, whereby alternative energization of the electromagnets and the drive motors will cause a walking motion of the carriage when attached to the ferro-magnetic surface, the device incorporating rotatable scrubbers for removing aquatic growths from that surface.
Lever et al U.S. Pat. No. 5,048,445 discloses providing a fluid jet system for underwater maintenance of a ship hull. The fluid jet system includes an open frame cart having a high pressure fluid nozzle manifold for cleaning and smoothing the submerged hull of the ship. One or more thruster assemblies are provided on the cart for deploying the cart through the water, advancing the cart along the hull and maintaining the cart in contact with the hull. Control of the thruster assembly and fluid flow manifold can be effected from either longitudinal end of the cart. Flexible fluid flow lines interconnect the cart to one or more remote sources of pressurized fluid so that the cart is independently operable. A system for deploying the cart is further provided and includes the necessary high pressure pumps, devices for hose deployment and retrieval, and diver supplies. Finally, a system of underwater maintenance of ship performance is provided whereby the condition of the hull of the ship is monitored and areas to be cleaned and smoothed are determined in order of priority based upon projected improvement to ship performance.
Hirana U.S. Pat. No. 4,697,536 provides for vessels and the like which require cleaning either periodically or whenever a need arises to remove various living things such as seaweeds and shells or contaminants such as oil for the sake of appearance and proper performance. Divers were conventionally employed to manually remove them one by one using a scraper as one means to remove such substances. Such manual operation is, however, extremely inefficient, involving great amounts of time and labor especially for large ships. The main body of a cleaning apparatus is pressed against an underwater object to be cleaned by means of impellers which are provided substantially at the center of the main body and driven to rotate, whereby cleaning brushes which are provided at the bottom of the cleaning apparatus concentrically with the impellers are rotated to remove substances adherent to the object while the cleaning apparatus is manipulated to run on the object's surface. Two pairs of an impeller and a brush are provided in parallel at the normal angle to the direction of forward and backward movement of the cleaning apparatus. The impellers and the brushes are driven by the same driving source as they are connected to the impellers by means of a universal joint.
Urakami U.S. Pat. No. 4,809,383 discloses a device capable of adhering to a wall surface by suction by the pressure of an ambient fluid and treating the wall surface, which comprises a pressure receiver member and a partition defining a pressure reduction zone in cooperation with the pressure receiver member and the wall surface. In one aspect, the partition has a sealing function of preventing inflow of a large amount of an outside fluid into the pressure reduction zone, and a treating function of treating the wall surface by being moved in a required manner. In another aspect, the partition has the above sealing function, and a travelling function of moving the device, by being rotated about an axis of rotation slightly inclined to an axis which is substantially perpendicular to the wall surface.
Okita et al U.S. Pat. No. 4,574,772 discloses an underwater cleaning apparatus having a carrier, a plurality of wheels for shifting the position of the carrier along a submerged surface, a plurality of rotary brushes carried by the carrier and adapted to clean the submerged surface, and a source of power for rotating the rotary brushes. The apparatus further comprises flexible partition wall members for transmitting torque to the rotary brushes and forming reduced pressure chambers communicated with spaces formed by bristles of respective rotary brushes. As the rotary brushes rotate, the rotary brushes and the partition wall members in combination serve to provide vacuum to produce a force to press the carrier through the wheels onto the submerged surface to be cleaned. In addition, each of the rotary brushes are allowed to individually follow the configuration of the surface thanks to the flexibility of the partition wall members.
In view of the substantial cost and time savings afforded by maintaining the submerged surfaces of ships in as smooth a condition as possible and by avoiding frequent dry docking and in view of the problems with previous systems for ship hull paint removal, it is an object of the invention to provide a system and method for removing coatings from the metal substrate of a hull.